For example, when manufacturing steel from steel scrap including scrap of galvanized steel sheets in a steelmaking electric furnace, the amount of dust produced during refining ranges from 13 to 17 kg per ton of molten steel, thus giving a huge annual production. This dust has a chemical composition as shown in the following Table 1.
TABLE 1 ______________________________________ (wt. %) T.Fe Zn Pb Cd C Cl F Na K ______________________________________ 20 10 1 0.1 0.5 2 0.3 1 0.5 to 40 to 30 to 6 to 0.5 to 1.5 to 5 to 1.0 to 4 to 3.0 ______________________________________
As shown in Table 1, the dust contains, in addition to iron, such useful metals as zinc, lead and other metals in the form of oxides in large quantities, and this dust has often been rejected as waste. However, to reject the dust with the above-mentioned chemical composition is very uneconomical from the point of view of effectively utilizing resources. Furthermore, since the aforementioned useful metals are toxic substances on the other hand, rejection of the dust having the above-mentioned chemical composition is a serious problems in environment control.
For these reasons, studies have been carried out actively in various circles concerned to find a method for recovering such useful metals as zinc, lead and other metals from a dust containing principally ferric oxide, zinc oxide and lead oxide, and as a result, the reducing volatilization process by rotary kiln has been industrialized as a relatively easy method.
With regard to the conventional reducing volatilization proces by rotary kiln as described above, the following proposal is made:
(1) A method for treating a dust discharged from a steelmaking furnace, disclosed in Japanese Patent Provisional publication No. 52,111/74 dated May 21, 1974, which comprises:
Charging a dust discharged from a steelmaking furnace, together with a solid carbonaceous reducing agent, into a rotary kiln, evaporating by reduction zinc oxide and lead oxide contained in the dust by heating the dust in the rotary kiln to separate zinc and lead from the dust; discharging from the rotary kiln, zinc and lead thus separated from the dust, together with exhaust gases produced in the rotary kiln, and recovering zinc and lead; carrying out a primary treatment comprising recovering iron powder through magnetic separation of clinker containing reduced iron after recovery of zinc and lead; then, carrying out a secondary treatment comprising charging again non-magnetic substances produced during said primary treatment into the rotary kiln and recovering zinc and lead remaining in said non-magnetic substances; recovering non-magnetic substances, principally comprising carbon produced during said secondary treatment; and then, using said non-magnetic substances thus recovered as a reducing agent (hereinafter referred to as the "prior art").
However, the aforementioned prior art involves the following problems.
(1) Recovery efficiency of useful metals from the dust is low.
(2) As a result of the low recovery efficiency of useful metals from the dust, a solid carbonaceous reducing agent is required in an amount of from 25 to 30 wt. % of the dust, and industrial treatment of the dust in a large quantity requires a large-capacity rotary kiln.
(3) Therefore, high running and installation costs are required for recovering useful metals from the dust.
Under such circumstances, there is a strong demand for the development of a method for efficiently recovering such useful metals as zinc, lead and other metals from a dust containing principally ferric oxide, zinc oxide and lead oxide discharged from a metal refining metallurgical furnace, which method requires only low running and installation costs. However, such a method is not as yet proposed.